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Regional Sequence-Stratigraphic Correlation and Basin Analysis of the Lower to Middle Triassic Moenkopi and Equivalent Formations, AZ, CA, UT, NV: Insight Into Backarc Basin Formation

Abstract

Ancient backarc basins have a poor preservation potential due to their position adjacent to active convergent margins. Current models are based on a handful of ancient backarc basins, thus their temporal evolution is not well constrained. A better appreciation of these basins can offer insight into poorly explored petroleum systems along modern and ancient convergent plate boundaries. The Lower to Middle Triassic Moenkopi Formation and equivalent units (Lower Mbr of the Buckskin Fm.), record the spatial and temporal transition from backarc to foreland basin sedimentation across AZ, CA, NV, NM, UT. This study constrains the location and evolution of sub-basins for the Moenkopi and Buckskin fms. by applying sequence-stratigraphic and basin analytic techniques. The Moenkopi Formation is interpreted as alluvial deposition in the east (AZ, NM, and UT) and marine to marginal-marine deposition in the west (NV, UT, AZ), the latter of which was deposited into a flexural foreland basin associated with the P-Tr Sonoma Orogeny. Depositional systems, determined from 9 newly measured and 100 published stratigraphic sections, delineated five third-order depositional sequences that were correlated over a >1500 km transect. Isopach maps for the two oldest sequences (Induan) highlight the development of the Paradox basin, whereas the maps for the three youngest sequences (Olenekian to Anisian) point out the development of numerous, isolated sub-basins that are then replaced by one broad depocenter in central AZ, which represents the transition from segmented sub-basins to a contiguous foreland basin sequence. Recent workers, who interpreted the Buckskin Fm. as marginal-marine, also noted depocenters in central AZ in the proximal backarc region of the P-Tr Cordilleran magmatic arc. A more detailed analysis of the Lower Member of the Buckskin Formation shows deep offshore marine deposition in a backarc setting parallel to the axis of the incipient Cordilleran magmatic arc. Consequently, the Moenkopi and equivalent fms. record the development of isolated sub-basins and their subsequent transition to a contiguous foreland basin system.